FIX: invalid syntax in elsif in Theme importer

elsif had a check that assigned to vars, this is not valid syntax and
could explode at runtime

lib/stylesheet/importer.rb

@@ -207,7 +207,9 @@ module Stylesheet
         end
       elsif callback = Importer.special_imports[asset]
         instance_eval(&callback)
-      elsif (path, source = match_theme_import(asset, parent_path))
+      else
+        path, source = match_theme_import(asset, parent_path)
+        if path && source
           Import.new(path, source: source)
         else
           Import.new(asset + ".scss")
@@ -215,3 +217,4 @@ module Stylesheet
       end
     end
   end
+end